Thermoplastic resin porous membranes have been widely used, for example, as materials for separation, selective permeation, and isolation of substances: e.g., battery separators used in a lithium ion secondary battery, nickel-hydrogen battery, nickel-cadmium battery, and polymer battery; separators for an electric double layer capacitor; various filters such as a reverse osmosis filtration membrane, ultrafiltration membrane, and microfiltration membrane; moisture-permeable waterproof clothing; and medical materials. In particular, polyethylene porous membranes have been suitably used as separators for a lithium ion secondary battery, because they are not only characterized by having excellent electrical insulating properties, having ion permeability due to electrolyte impregnation, and having excellent electrolyte resistance and oxidation resistance, but also have the pore-blocking effect of blocking a current at a temperature of about 120 to 150° C. in abnormal temperature rise in a battery to suppress excessive temperature rise. However, if the temperature continues to rise for some reason even after pore blocking, membrane rupture can occur at a certain temperature as a result of decrease in viscosity of molten polyethylene constituting the membrane and shrinkage of the membrane. In addition, if the membrane is left at a constant high temperature, membrane rupture can occur after the lapse of a certain time as a result of decrease in viscosity of molten polyethylene and shrinkage of the membrane. This phenomenon is not a phenomenon that occurs only when polyethylene is used, and also when any other thermoplastic resin is used, this phenomenon is unavoidable at or higher than the melting point of the resin constituting the porous membrane.
In particular, separators are highly responsible for battery properties, battery productivity, and battery safety, and required to have excellent mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking properties (shutdown properties), melt rupture properties (meltdown properties), and the like. Further, lithium ion secondary batteries, upon demand for cost reduction in recent years, are strongly required to be produced more efficiently. Therefore, it is expected that separators for a lithium ion secondary battery will, in the future, increasingly require higher processability (electrolyte permeability, low curling properties) in a battery assembly process.
In particular, any improvement in electrolyte permeability contributes significantly to battery productivity, which is of extremely great value.
In recent years, techniques using lamination of a heat resistant resin layer on a polyolefin separator membrane have been proposed. Lamination of a heat resistant resin having high affinity for electrolyte solutions improves electrolyte permeability to some degree.
Coating a polyolefin porous membrane with a coating solution containing a heat resistant resin and immersing a polyolefin porous membrane in a coating solution containing a heat resistant resin are a common method for laminating the heat resistant resin layer as described above on a polyolefin porous membrane. However, these methods have a problem of air resistance increase due to clogging of pores of the polyolefin porous membrane caused by lamination of the heat resistant resin. If a polyolefin porous membrane with a larger pore size is used in order to reduce the clogging of pores, an important pore-blocking function, which determines the safety of a separator, will be reduced.
To satisfy these requirements, various studies to improve heat resistance have hitherto been conducted.